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linux 线程的同步 二 (互斥锁和条件变量)
为了允许在线程或进程之间共享数据,同步时必须的,互斥锁和条件变量是同步的基本组成部分。
1、互斥锁
互斥锁是用来保护临界区资源,实际上保护的是临界区中被操纵的数据,互斥锁通常用于保护由多个线程或多进程分享的共享数据。一般是一些可供线程间使用的全局变量,来达到线程同步的目的,即保证任何时刻只有一个线程或进程在执行其中的代码。一般加锁的轮廓如下:
pthread_mutex_lock()临界区pthread_mutex_unlock()
互斥锁API
pthread_mutex_lock(pthread_mutex_t *mutex);
用此函数加锁时,如果mutex已经被锁住,当前尝试加锁的线程就会阻塞,直到互斥锁被其他线程释放。当此函数返回时,说明互斥锁已经被当前线程成功加锁.
pthread_mutex_trylock(pthread_mutex_t *mutex);
用此函数加锁时,如果mutex已经卑琐主,当前尝试加锁的线程不会阻塞,而是立即返回,返回的错误码为EBUSY,而不是阻塞等待。
pthread_mutex_unlock(pthread_mutex_t *mutex);
注意使用锁之前要记得初始化。互斥锁的初始化有两种初始化方式:
1.对于静态分配的互斥锁一半用宏赋值的方式初始化
eg: static pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
2.对于动态分配的互斥锁(如调用malloc)或分配在共享内存中,则必须调用pthread_mutex_init(pthread_mutex *mutex, pthread_mutexattr_t *mutexattr)函数来进行初始化。
例子1:写个程序实现生产者—消费者问题,先只考虑多个生产者线程之间的同步,直到所有的生产者线程都完成工作以后,才启动消费者线程。程序如下:
1 #include <stdio.h> 2 #include <stdlib.h> 3 #include <unistd.h> 4 #include <pthread.h> 5 #include <errno.h> 6 7 #define MAXNITEMS 1000000 8 #define MAXNTHREADS 100 9 10 int nitems;11 12 struct13 {14 pthread_mutex_t mutex;15 int buff[MAXNITEMS];16 int nput;17 int nval;18 } shared = {19 PTHREAD_MUTEX_INITIALIZER20 };21 22 void *produce(void*);23 void *consume(void*);24 25 int main(int argc,char *argv[])26 {27 int i,nthreads,count[MAXNTHREADS];28 pthread_t tid_produce[MAXNTHREADS],tid_consume;29 if(argc != 3)30 {31 printf("usage: producongs2 <#itmes> <#threads>.\n");32 exit(0);33 }34 nitems = atoi(argv[1]);35 nthreads = atoi(argv[2]);36 pthread_setconcurrency(nthreads); //设置线程并发级别37 for(i=0;i<nthreads;++i)38 {39 count[i] = 0;40 pthread_create(&tid_produce[i],NULL,produce,&count[i]);41 }42 for(i=0;i<nthreads;i++)43 {44 pthread_join(tid_produce[i],NULL); //等待线程退出45 printf("count[%d] = %d\n",i,count[i]);46 }47 pthread_create(&tid_consume,NULL,consume,NULL);48 pthread_join(tid_consume,NULL); //等待线程退出49 exit(0);50 }51 52 void *produce(void *arg)53 {54 for(; ;)55 {56 pthread_mutex_lock(&shared.mutex); //加锁57 if(shared.nput >= nitems)58 {59 pthread_mutex_unlock(&shared.mutex); //释放锁60 return ;61 }62 shared.buff[shared.nput] = shared.nval;63 shared.nput++;64 shared.nval++;65 pthread_mutex_unlock(&shared.mutex); //加锁66 *((int*) arg) += 1;67 }68 }69 void *consume(void *arg)70 {71 int i;72 for(i=0;i<nitems;i++)73 {74 if(shared.buff[i] != i)75 printf("buff[%d] = %d\n",i,shared.buff[i]);76 }77 return;78 }
程序执行结果如下:
例子2:改进例子1,所有生产者线程启动后立即启动消费者线程,这样生产者线程产生数据的同时,消费者线程就能出来它,此时必须同步生产者和消费者,程序如下:
1 #include <stdio.h> 2 #include <stdlib.h> 3 #include <unistd.h> 4 #include <pthread.h> 5 #include <errno.h> 6 7 #define MAXNITEMS 1000000 8 #define MAXNTHREADS 100 9 10 int nitems;11 12 struct13 {14 pthread_mutex_t mutex;15 int buff[MAXNITEMS];16 int nput;17 int nval;18 } shared = {19 PTHREAD_MUTEX_INITIALIZER20 };21 22 void *produce(void*);23 void *consume(void*);24 void consume_wait(int);25 int main(int argc,char *argv[])26 {27 int i,nthreads,count[MAXNTHREADS];28 pthread_t tid_produce[MAXNTHREADS],tid_consume;29 if(argc != 3)30 {31 printf("usage: producongs2 <#itmes> <#threads>.\n");32 exit(0);33 }34 nitems = atoi(argv[1]);35 nthreads = atoi(argv[2]);36 pthread_setconcurrency(nthreads+1);37 //创建生产者线程38 for(i=0;i<nthreads;++i)39 {40 count[i] = 0;41 pthread_create(&tid_produce[i],NULL,produce,&count[i]);42 }43 //创建消费者线程44 pthread_create(&tid_consume,NULL,consume,NULL);45 for(i=0;i<nthreads;i++)46 {47 pthread_join(tid_produce[i],NULL);48 printf("count[%d] = %d\n",i,count[i]);49 }50 //等待消费者线程退出51 pthread_join(tid_consume,NULL);52 exit(0);53 }54 55 void *produce(void *arg)56 {57 for(; ;)58 {59 pthread_mutex_lock(&shared.mutex);60 if(shared.nput >= nitems)61 {62 pthread_mutex_unlock(&shared.mutex);63 return ;64 }65 shared.buff[shared.nput] = shared.nval;66 shared.nput++;67 shared.nval++;68 pthread_mutex_unlock(&shared.mutex);69 *((int*) arg) += 1;70 }71 }72 void *consume(void *arg)73 {74 int i;75 for(i=0;i<nitems;i++)76 {77 consume_wait(i);78 if(shared.buff[i] != i)79 printf("buff[%d] = %d\n",i,shared.buff[i]);80 }81 return;82 }83 void consume_wait(int i)84 {85 for(; ;) //进行轮询,判断i是否已经由生产者生产86 {87 pthread_mutex_lock(&shared.mutex);88 if(i<shared.nput) //i已经生产89 {90 pthread_mutex_unlock(&shared.mutex);91 return; 92 }93 pthread_mutex_unlock(&shared.mutex);94 }95 }
存在的问题:当消费者获取的条目尚没有准备好时,消费者线程一次次的循环去判断,每次给互斥锁解锁又上锁,这种轮询的办法浪费CPU时间。
2、条件变量
互斥锁用于上锁,条件变量用于等待,条件变量的使用是与互斥锁共通使用的。
2.1等待与信号发送
条件变量类型是pthread_cond_t,调用函数如下:
pthread_cond_wait(pthread_cond_t *cond, pthread_mutex_t *pmutex);
pthread_cond_signal(pthread_cond_t *pcond);
每个条件变量总是有一个互斥锁与之关联。现在采用条件变量实现生产者与消费者问题,程序如下:
1 #include <stdio.h> 2 #include <stdlib.h> 3 #include <unistd.h> 4 #include <pthread.h> 5 #include <errno.h> 6 7 #define MAXNITEMS 1000000 8 #define MAXNTHREADS 100 9 10 int nitems;11 12 struct13 {14 pthread_mutex_t mutex;15 int buff[MAXNITEMS];16 int nput;17 int nval;18 } shared = {19 PTHREAD_MUTEX_INITIALIZER20 };21 //条件变量22 struct {23 pthread_mutex_t mutex; 24 pthread_cond_t cond;25 int nready;26 }nready = {27 PTHREAD_MUTEX_INITIALIZER,PTHREAD_COND_INITIALIZER28 };29 30 void *produce(void*);31 void *consume(void*);32 33 int main(int argc,char *argv[])34 {35 int i,nthreads,count[MAXNTHREADS];36 pthread_t tid_produce[MAXNTHREADS],tid_consume;37 if(argc != 3)38 {39 printf("usage: producongs2 <#itmes> <#threads>.\n");40 exit(0);41 }42 nitems = atoi(argv[1]);43 nthreads = atoi(argv[2]);44 pthread_setconcurrency(nthreads+1);45 for(i=0;i<nthreads;++i)46 {47 count[i] = 0;48 pthread_create(&tid_produce[i],NULL,produce,&count[i]);49 }50 pthread_create(&tid_consume,NULL,consume,NULL);51 for(i=0;i<nthreads;i++)52 {53 pthread_join(tid_produce[i],NULL);54 printf("count[%d] = %d\n",i,count[i]);55 }56 pthread_join(tid_consume,NULL);57 exit(0);58 }59 60 void *produce(void *arg)61 {62 printf("producer begins work\n");63 for(; ;)64 {65 pthread_mutex_lock(&shared.mutex);66 if(shared.nput >= nitems)67 {68 pthread_mutex_unlock(&shared.mutex);69 return ;70 }71 shared.buff[shared.nput] = shared.nval;72 shared.nput++;73 shared.nval++;74 pthread_mutex_unlock(&shared.mutex);75 pthread_mutex_lock(&nready.mutex);76 if(nready.nready == 0)77 pthread_cond_signal(&nready.cond); //通知消费者78 nready.nready++;79 pthread_mutex_unlock(&nready.mutex);80 *((int*) arg) += 1;81 }82 }83 void *consume(void *arg)84 {85 int i;86 printf("consuemer begins work.\n");87 for(i=0;i<nitems;i++)88 {89 pthread_mutex_lock(&nready.mutex);90 while(nready.nready == 0)91 pthread_cond_wait(&nready.cond,&nready.mutex); //等待生产者92 nready.nready--;93 pthread_mutex_unlock(&nready.mutex);94 if(shared.buff[i] != i)95 printf("buff[%d] = %d\n",i,shared.buff[i]);96 }97 return;98 }
程序执行结果如下:
总的来说,给条件变量发送信号的过程代码如下:
struct{ pthread_mutex_t mutex; pthread_cond_t cond; //维护本条件的各个变量}var = {PTHREAD_MUTEX_INITIALIZER,PTHREAD_COND_INITIALIZER,...}pthread_mutex_lock(&var.mutex);设置条件为真pthread_cond_signal(&var.cond);pthread_mutex_unlock(&var.mutex);
测试条件并进入睡眠以等待条件变为真的代码大体如下:
pthread_mutex_lock(&var.mutex);while(条件为假) pthread_cond_wait(&var.cond,&var.mutex);修改条件pthread_mutex_unlock(&var.mutex);
2.2定时等待和广播
通常pthread_cond_signal只是唤醒等待在相应条件变量上的一个线程,在某些情况下需要唤醒多个线程(例如读写者问题),可以调用pthread_cond_broadcast唤醒阻塞在相应条件变量上的所有线程。pthread_cond_timewait允许线程就阻塞时间设置一个限制值。API如下:
pthread_cond_broadcast(pthread_cond_t *cond);
pthread_cond_timedwait(pthread_cond_t *cond, pthread_mutex, const struct timespec *abstime);
linux 线程的同步 二 (互斥锁和条件变量)